1. Field of the Invention
The invention generally concerns methods for the synthesis of dialkyl carbonates, and more specifically dimethyl carbonate (DMC) and diethyl carbonate (DEC).
2. Description of the Prior Art
Methyl tertiary-butyl ether (MTBE) is a compound used almost exclusively as a fuel additive in gasoline. MTBE is a member of a class of chemicals known as xe2x80x9coxygenatesxe2x80x9d because they raise the oxygen content of gasoline. MTBE has been used in the United States in low levels as a gasoline additive since the late 1970""s to replace lead as an octane enhancer. Since the early 1990""s, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements of the Clear Air Act Amendments. However, MTBE has been detected in groundwater throughout the United States. MTBE can make drinking water supplies undrinkable due to its offensive taste and odor. While there is limited evidence on the hazards MTBE poses to human health, animal studies have shown MTBE to be a carcinogen.
Because of the environmental problems and potential health risks associated with the use of MTBE, there has been a push to replace MTBE as the oxygenate additive in gasoline. Dialkyl carbonates, especially dimethyl carbonate (DMC) and diethyl carbonate (DEC), have been proposed to replace MTBE as oxygenate gasoline additives because dialkyl carbonates have been shown to be more environmentally friendly and pose fewer health risks than MTBE.
The most traditional method of making dialkyl carbonates has been to react an alcohol with phosgene. This method poses numerous safety hazards derived mainly from the use of phosgene which is highly toxic. Therefore, it was desirable to develop a method of synthesizing dialkyl carbonates without using phosgene.
Alternate methods of synthesizing dialkyl carbonates were developed relying heavily on the use of catalysts in reacting alcohols and carbon monoxide. U.S. Pat. No. 3,114,762 discloses one such method for producing organic carbonates by reacting ethanol and carbon monoxide in the presence of a palladium or platinum catalyst to produce DEC. These catalysts are often expensive and require regeneration from time to time.
U.S. Pat. No. 4,113,762 discloses a method of synthesizing dialkyl carbonates by reacting an alcohol with carbon monoxide and oxygen in the presence of a copper-containing catalyst. However copper-containing catalysts also have certain drawbacks, such as sensitivity to water, which tend to reduce the reaction rate and selectivity of carbon monoxide towards the formation of the dialkyl carbonate.
U.S. Pat. No. 5,118,818 discloses a process for preparing an organic carbonate by directly reacting an alcohol or diol, carbon monoxide, a halogen, and a halide ion. This process is deficient in that it involves endothermic reactions thereby requiring the input of energy in order to drive the reactions toward the desired products. From an economic viewpoint, endothermic reactions are typically not as desirable as exothermic reactions because endothermic reactions proceed at a much slower rate and require that energy be input into the system.
The current invention provides methods of forming dialkyl carbonates which do not use phosgene as an initial reactant, do not require the use of expensive metal catalysts, and are exothermic in nature thereby eliminating the necessity for providing substantial energy input in driving the reactions toward the desired end products.
Generally, methods according to the invention for forming a dialkyl carbonate having the formula 
wherein R is a C1-C15 alkyl, alkylene (an organic radical formed from unsaturated aliphatic hydrocarbons), aryl, or aralkyl radical comprise reacting an alcohol or diol having the formula Rxe2x80x94OH or HOxe2x80x94Rxe2x80x94OH, a base, and a halogen (X) in the presence of an amine salt forming a first intermediate having the formula Rxe2x80x94Oxe2x80x94X. The first intermediate is reacted with carbon monoxide thereby forming a second intermediate having the formula 
and the second intermediate is reacted with the alcohol or diol in the presence of the amine salt thereby forming the dialkyl carbonate.
In preferred embodiments of the invention, the R radical of the alcohol or diol is selected from the group consisting C1-C8 alkyl, alkylene, aryl or aralkyl radicals. However, preferably the alcohol is a C1-C4 alcohol and even more preferably, the alcohol is selected from the group consisting of methanol, ethanol, and tertbutanol.
Preferably, the halogen for use with the invention is selected from the group consisting of chlorine, bromine, or iodine. Most preferably the halogen is chlorine. The halogen for use with the invention can be in either solid, liquid or gaseous state, however a gas is preferred.
The base used in the reaction can be any base selected from the group consisting of alkali metal salts, alkaline-earth metal salts, tertiary amines, and pyridine. Preferably the base is an alkali or alkaline-earth metal salt selected from the group consisting of alkali and alkaline-earth metal carbonates, bicarbonates, hydroxides, and sulfates. Most preferably, the base will be selected from the group consisting of sodium carbonate, calcium carbonate, magnesium carbonate, sodium hydroxide, calcium hydroxide, and magnesium hydroxide.
The amine salt used with the invention acts as a catalyst in facilitating the ultimate conversion of the reactants into the dialkyl carbonate product. Preferably the amine salt for use with the invention has the formula 
where R1, R2, R3, and R4 are individually selected from the group consisting of H, C1-C22 straight or branched, substituted or unsubstituted alkyl, alkylene (any unsaturated hydrocarbon radical), aryl, or aralkyl radicals, and X is a halide ion. More preferably, the amine salt will comprise a quaternary ammonium salt, and even more preferably comprises a C12-C15 alkyl trimethylammonium chloride salt, wherein R1, R2, and R3 are each a methyl radical and R4 will comprise a C12-C15 straight or branched, substituted or unsubstituted alkyl radical.
In preferred embodiments, the base is present in an amount between about 0.01-25% by weight based on the weight of the alcohol being 100%. Preferably the amine salt is present in an amount between about 0.01-25% by weight based on the weight of the alcohol being 100%.
The dialkyl carbonate product may be separated from the reaction mixture by any means known to those skilled in the art. However, distillation is the preferred method of isolating the dialkyl carbonate product.
The following series of chemical equations depict the probable reaction scheme and mechanism employed with the invention and should not be taken as the exclusive reaction scheme and mechanism and nothing therein should be taken as a limitation upon the overall scope of the invention.
For illustrative purposes only, in the following series of equations, the reactants are an alcohol having the formula Rxe2x80x94OH, sodium carbonate (Na2CO3), elemental chlorine (Cl2), and carbon monoxide (CO). A diol may be substituted for the alcohol in the equations, however for the sake of simplicity, only the use of an alcohol will be shown. Likewise, any halogen may be substituted for chlorine. The amine salt catalyst used is an alkyl trimethylammonium chloride salt (Rxe2x80x2xe2x80x94NMe3).
The overall reaction scheme comprises three general steps. In the first step, the 
alcohol and chlorine are reacted in the presence of the base and amine salt to form a first intermediate having the formula Rxe2x80x94Oxe2x80x94Cl.
The second step comprises the reaction of the first intermediate, Rxe2x80x94Oxe2x80x94Cl, with carbon monoxide to form a second intermediate comprising an alkyl chloroformate. Lastly, the second intermediate quickly reacts with the alcohol in the presence of the base and amine salt to form the final dialkyl carbonate. 
The probable reaction mechanism for the above steps is set forth below. The reaction mechanism generally comprises four steps. In the first step, the hydrogen of 
the hydroxyl group of the alcohol is removed. 
The second step involves the formation of the first intermediate (alkyl hypochloride) described above.
The third step involves the formation of the second intermediate (alkyl chloroformate) described above.
The fourth step involves the reaction of the second intermediate to form the final dialkyl carbonate product. 